1. Field of the Invention
The present invention relates to a liquid crystal material for use in a thermal writing liquid crystal element utilizing the thermo-optic effect of liquid crystals.
2. Description of the Prior Art
When some portions of a thin layer of cholesteric phase or smectic phase liquid crystal showing a transparent texture are heated and then rapidly cooled, such portions are in general changed to a phase of a light scattering opaque texture. Such phenomenon is known as a thermo-optic effect of liquid crystals.
Some proposals have been made to employ present liquid crystal elements for writing information therein by utilizing the above phenomenon, that is, by giving partial temperature changes to a liquid crystal cell to alter the partial portions opacity. For example, direct observation type display elements to observe the direct video image as written, light valve elements for use in projection type display to observe, by projection, the video image as written, recording elements to read out the written informations therefrom by electric or optic means, etc. have been proposed. There are also some known methods of writing informations in the elements, for example, a method to radiate a laser beam thereby to elevate the temperature of the irradiated portions, a method to cause temperature elevation by means of Joule heating at resistors, etc. Only for the purpose of simplification, the present invention will be explained hereinafter with respect to an example limited to the application to a light valve element which is used in a projection type display unit for writing a video image in a liquid crystal cell by radiation of laser beam and observing the written video image as enlarged and projected by means of a separate optical system.
At an early stage, liquid crystal compounds used for the thermal writing light valves were of what is called "Schiff's base" series represented by the formula (1): ##STR4##
It is because no compounds other than those of the Schiff's base series had been found to have the properties required to the liquid crystal materials for light valves, that is, to show smectic phase at the operation temperatures, namely around the room temperatures, and to have a suitable phase transition temperature. The suitable phase transition temperature relates to the temperature Tc at which the liquid crystal changes into the isotropic liquid phase. Too high Tc requires greater elevation of the temperature for writing which is not advantageous in respect of the required electric power and the writing speed. On the other hand, too low Tc brings about lower contrast. One of the reasons for such lower contrast is that the order parameter of the alignment of the liquid crystal molecules is lowered owing to the fact that the temperature of the liquid crystal at the steady state after writing is near to the phase transition temeperature Tc. Such liquid crystal compounds as above of the Schiff's base series are reported by Gary N. Taylor, et al. of Bell Laboratories in the "Journal of Applied Physics", Vol. 45, No. 10, October 1974, pages 4330-4338.
It is however widely known that these liquid crystal compounds of Schiff's base series are readily hydrolysed and so the thermal writing elements made of such compounds have short lifetimes particularly in respect of the humidity resistance and thus are lacking in the reliability. Under the circumstances, attempts have been made to provide improved liquid crystal materials and thus chemically stable liquid crystal compounds of biphenyl series of the formula (2): ##STR5## wherein R means an alkyl or alkoxy radical have been produced. In the report made by A. G. Dewey of IBM Research Laboratory in "The Physics and Chemistry of Liquid Crystal Devices" published from Plenum Publishing Corp. (1980) at pages 219-239, it is described that the mixture of three components each selected from the group of the liquid crystal compounds having the constitution of said formula (2) wherein R stands for C.sub.8 H.sub.17, C.sub.9 H.sub.19, C.sub.10 H.sub.21, C.sub.11 H.sub.23, C.sub.8 H.sub.17 O or C.sub.9 H.sub.19 O is most suitable for use in thermal writing light valves. Further, according to the report by J. C. Dubois of Thomson-CSF, Laboratoire Central de Recherches in "Annales de Physique" (1978) v. 3, at pages 131-138, the comparison and study of the mixtures of components selected from the group of the liquid crystal compounds having the constitution of the formula (2'): ##STR6## wherein R' stands for C.sub.8 H.sub.17, C.sub.9 H.sub.19, C.sub.3 H.sub.7 O, C.sub.8 H.sub.17 O, C.sub.8 H.sub.17 COO or C.sub.9 H.sub.19 COO have revealed that the mixture of three components each corresponding to the case R'=C.sub.8 H.sub.17, C.sub.9 H.sub.19 or C.sub.9 H.sub.19 COO is most suitable for use in the thermal writing light valves. Therefore some improvements have been achieved and the defects of short lifetime, low reliability, etc. as accompanied in the case of early found Schiff's base series liquid crystal compounds have been overcome to some extent.
It has been noted however that the above mentioned known liquid crystal materials are still insufficient particularly in respect of the liquid crystal phase temperature ranges. For instance, the mixture of the compounds of the formulae (3) (4): ##STR7## as recommended by Dewey as above has the phase transition temperature Tm between crystal and liquid crystal of about 12.degree. C., and the mixture of the three compounds of the formulae (3) (5) (6): ##STR8## as recommended by Dubois as above has Tm of 8.degree. C. The liquid crystal light valve made from such materials as above crystallizes when it is cooled to the temperature Tm, that is, around 10.degree. C. or lower. After crystallization, it naturally does not operate and, owing to such crystallization history, it has a defect in its liquid crystal molecule alignment even after restoration of the temperature, which defect causes disadvantage of deteriorating video quality of display. In practice, it is extremely inconvenient that the lower limit of the storage temperature is restricted to an extent of 10.degree. C. and there is a strong practical demand for the temperature around -10.degree. C., as being widely known. The actual circumstances are as mentioned above and thus no satisfactory light valves can be obtained by the use of the known liquid crystal materials.
The present inventor has investigated in detail the relation between the transition temperature and the display performance of the liquid crystal materials and conducted experiments and studies on a great number of liquid crystal materials. As the result, it has successfully been found that certain mixtures of liquid crystal compounds have wide temperature range of liquid crystal phase and made it possible to provide elements having a high performance that could not hithereto be obtained. The present invention thus has been accomplished.